The present invention generally relates to metal casting foundry operations, and more particularly, to methods for forming sand cores, sand molds or other molding elements for use in metal casting foundry operations.
Sand molds typically comprise upper and lower shells (often referred to as a cope and a drag) which provide a hollow internal compartment therebetween to form the external shape of a simple metal casting. Frequently it is desired that the metal casting contains an internal cavity, such as a fluid passageway for example. Anyone can see at a glance that a metal casting cavity contains nothing. As such, special forms known as sand cores are used to shape the interior design of a metal casting. The core, thus, merely defines the shape during molten metal filling by preventing the flowing metal from occupying this space. After the mold has solidified, the core is destroyed at shakeout, leaving only the correctly shaped casting cavity. The sand mold including the cope and the drag, as well as internal cores can be generally characterized as molding elements.
A foundryman can also use a core to shape the external part of the more intricate casting. For instance, if a section of the casting is an undercut, a core can be used for section, so that the pattern can be withdrawn from the mold without distorting the mold. Besides forming internal cavities surrounded by metal, or some external surfaces of an intricate casting, a core is sometimes used to strengthen or improve a particular inner or outer surface of the mold.
Some of the typical requirements of molding elements are that they are workable in moldings and have sufficient bench life, that they are able to vent off gases during molten metal pouring and cooling operations, and that they are able to have good collapsibility such that the sand shakes out well once the metal is cooled to expose the metal casting and any internal cavities in the metal casting. As such, copes, drags and cores are typically made of dry, free-flowing sand. Special binders are added to the sand to hold the sand together in the desired shape, and generally give the individual molding element its name. For example, the following types of sand/binder materials have been used for molding elements in metal casting foundry processes: green sand, hot box, oil bonded, furan (no bake), shell, cold box, sodium silicate CO2, and others.
In high volume production, foundrymen form the separate sand-shaped cores by compacting a special sand mixture in a core box. The core box is a specially designed structure, the cavity of which is shaped like the core to be made. Copes and drags are typically made in a special machine that includes a matchplate that occupies the internal hollow compartment between the cope and the drag while the mold is being formed.
While the above-described method for making molding elements works very well for high volume production, it has several drawbacks with regards to making single prototypes or low volume production of metal castings where turn around time is absolutely critical. Manufacturers who use metal castings are in constant competition to see who can get a new product to the marketplace the quickest. Manufacturers which have their new product to the marketplace first can gain a significant commercial advantage.
Manufacturers often make their decision on where to out source a cast metal prototype component based on the quoted turn around time. Conventional methods of casting production, including the construction of tooling and the pouring a casting, are often too time-exhaustive to provide a metal model to a company for verification of the component""s shape and its fit in the overall end product design. As such some manufacturers do not even create actual metal castings but simply rely on various other rapid prototyping methods that have evolved for design verification before a casting is poured. Rapid prototyping methods can make the process of xe2x80x9cprintlessxe2x80x9d tooling and casting quotation easier and more accurate. For example, various rapid prototyping processes such as Fused Deposition Modeling (FDM), Laminated Object Manufacturing (LOM), Selective Laser Sintering (SLS), Solid Ground Curing (SGC), Stereolithography (SLA), Three dimensional Printing (3DP), Direct Shell Production Casting (DSPC), have been used to create prototypes. However, these methods are still more time consuming than desired or have practicality limitations. Moreover, many of these methods only produce wax, plastic or paper/woodlike prototypes, which are insufficient for most laboratory testing. Even machining a metal prototype will yield a prototype with different precision qualities and strength qualities as compared with a cast metal prototype.
Despite the recent advances in computer-based simulations of casting solidification, many manufacturers still require prototypes of metal castings to be tested before approval is given for mass production. Moreover, actual metal castings are usually desired in any event due to the extreme costs of making changes after a design is released for production or even into the marketplace. For the foundry and the manufacturer, the verification of a cast metal component before full production is vital to reducing lead times and total costs. For example, the costs of changing the basic design of a product increase rapidly as the design advances through the development cycle. The development cycle can generally be categorized in the following five (5) steps, including: conceptual modeling, detailed design, prototype/test, manufacturing, and product release. Making changes during the conceptual modeling stage is by far the cheapest, while changes at the product release stage are by far the most expensive. It has been estimated that the cost of making changes increases tenfold for each different step during the process. For example, if a change was to be made during conceptual modeling which costs one dollar, the same change made during the prototype/test stage would be a hundred dollars, and at the manufacturing time, one thousand dollars, and at the product release stage, ten thousand dollars. Therefore, the quicker a prototype can be made for use in testing or verification, the more changes a manufacturer can make to a metal casting before it is manufactured or released into the marketplace.
The problem is that the production of hard tooling (such as metal dies for die casting, permanent molding, and investment wax for injection, or cope/drag tooling to which the long sand casting production runs) often do not meet the manufacturer""s desired lead time requirements, for example, a shipment of a hundred prototype castings for use in three (3) weeks. Even with these advances in rapid prototyping processes, manufacturers are still not satisfied with the requisite time it takes to obtain a cast metal prototype for use in testing, or a test market. The common method to form these metal casting prototypes is to use one of the rapid prototype patterns (for example a woodlike prototype made by the LOM process) that is durable enough to survive the sand molding process and can be used directly as a master pattern to make sand mold elements. Rapid prototype core boxes can be used to make the cores that are filled with sand manually, rather than with a core blower.
Currently, there is not a quick enough turnaround time in industry with regards to making metal castings. Moreover, precision can be lost in the making of sand molds from plastic, wax or woodlike prototypes and core boxes formed by from a wood-like, plastic or wax prototype or other appropriate prototype made via a rapid prototyping process. Moreover, the sand can settle or change shape after being formed in the core box and allowed to cure, which also slightly changes the shape of the core box and results in less precession. This also is likewise undesirable.
It is therefore an object of the present invention to overcome these and other disadvantages and problems existing out there in the art.
It is a specific object of the present invention to provide faster methods for forming prototype or low volume metal castings to provide quicker turn around times for prototype or low volume metal castings.
It is another object of the present invention to provide a faster way to form sand mold elements such as a cope, a drag, a core, or other such sand pattern type items for use in foundry molds to produce prototype or low volume metal castings.
It is another object of the present invention to provide a more precise way to form metal castings.
In accordance with these and other objectives, the present invention is directed at a method of machining blocks of sand into sand mold elements such as sand cores, copes and drags for use in foundry processes to form metal castings. The block of sand is held together by conventional foundry binder material and is machined with a machine tool to the desired pattern of the sand mold element. Preferably, the sand block is machined, using computer numerical control (CNC) machining to form the sand block into the desired pattern. CNC machining utilizes readily obtainable CAD files or other computer readable files that can be used for the design of the metal casting. It is an advantage that this eliminates the need to form woodlike paper, plastic, or wax prototypes. A significant advantage of the present invention is that the turnaround time and cost of producing metal castings is significantly reduced.
According to the preferred embodiment, a sand block is clamped into a machine tool such as a mill, a lathe, a drill, or other similar machine tool. The block may be clamped into a stationary position or may be adapted to move or rotate. A cutting tool is then used to remove sand from the sand block to form the desired mold element pattern. Preferably, the pattern is roughed out with a roughing tool, and then finished to a high degree of precision with a finishing tool. The sand block may be unclamped and reclamped if necessary to perform machining for multiple sides or to perform machining with multiple machine tools. Locating pins, may be used in the sand block to maintain alignment of the core when it is being unclamped and reclamped. All of the sand mold elements including the cope, the drag and the internal cores if necessary may be formed by this method. Once the sand mold elements are formed, the sand core is inserted into a prepared mold which includes a cope and a drag. The common use of a machined sand core is to form internal cavities such as fluid passageways and the like inside of the metal casting. The common use of machined sand copes and drags is to form the external surface of the metal casting. Cores may also be used to form external structural features. Once the sand mold is complete, molten metal is poured into the mold and allowed to cool. Once the metal is sufficiently cooled and hardened, the cope, the drag and the core are destroyed at shakeout, thereby leaving only the correctly shaped metal casting along with any internal cavities in the metal casting.